Tap changers are used in combination with power transformers for an interruption-free switchover between successive winding taps of this transformer, primarily for interruption-free voltage control.
Tap changers for this purpose in the past have generally operated in accordance with two principles primarily in different regions of the world:
1. The slow-switching reactor switch which is currently used in the United States and was used in part in the prior Soviet Union. In this case, switching impedances are provided which, during the slow switchover from one winding tap to the next, prevent short-circuiting of the stages of the transformer and must be dimensioned for the period in which they are under load.
2. Rapidly-acting switches which have been given the name of their inventor, namely, "Jansen switches" which are used in the remainder of the world. The switchover from one winding type to the next is effected rapidly, i.e. in a jump, and utilizes switching resistances which reduce or prevent a short circuit even for the very brief time interval that the switching requires.
This application refers to tap selector switching in accordance with the first-mentioned reactor switching principle.
A tap selector of this type is described in the brochure "Load Tap Changer Type RMV II" of the firm Reinhausen Manufacturing, Humbolt, Tenn., U.S.A., No. RM 05/91-1094/5000.
In this tap selector, vacuum switching cells are provided for switching under load. Vacuum switching cells have a number of advantages by comparison with mechanical load switching contacts, namely, a significantly higher operating life. Using such vacuum switching cells a contamination of the surrounding oil is completely prevented, such contamination readily arising with mechanical switch contacts which must operate under load and therefore tend to spark or suffer significant contact burn off.
In the specific description below, reference will be made to the sequence in which the vacuum switching cell and other switch contacts of the reactor type tap selector operate and for the present purposes it is only required to understand that generally speaking the tap selector switching can be subdivided into a stage A which can be referred to as the existing stage or previous stage and a neighboring stage B which can be referred to as the subsequent stage or the stage into which the tap selector is to be switched. While the switching will be described in a single phase, it will be understood that the transformers involved are generally three-phase transformers and a set of selector contacts is normally provided for each of the phases of the three phases and the three phases are switched together, i.e. the moving switch contacts are ganged for joint movement.
It is convenient to refer to the tap which has been previously selected as the tap n and the tap to be selected as the neighboring tap is n+1 for the tapped winding of the particular phase of the transformer.
A pair of selector contacts P1 and P2 can then be provided and in succession, will both engage the previously selected tap and be moved so that a leading one of these contacts engages the next tap. In a subsequent stage the trailing contact moves over to that next tap.
In series with the contacts P1 and P2, namely, the movable selector contact, are switching impedances which can be referred to as R1 and R2, the opposite ends of these impedances being bridged by a vacuum switching cell V and having a bypass switching B with movable contacts connecting the impedance ends to a leading line L.
In a stationary state of the system prior to a tap change operation, both movable contacts P1 and P2 engage the fixed tap contacts n, the vacuum switching cell is closed while the movable contacts of the bypass switch B are closed in preparation for the next phase. In the next phase one of the movable bypass contacts opens so that the load current flow is through the vacuum switching cell and the contact of the bypass switch will remain closed. The vacuum switching cell can then be opened, cutting off the impedance associated with the open bypass contact and hence that movable selector contact can be shifted into engagement with the next tap fixed contact. The vacuum switching cell is then closed to put the new tap under load, the previously opened bypass contact is closed and the process can be repeated with opening of the vacuum switching cell and the other bypass contact until the second movable contact has made the transition from the fixed previous tap contact to the next contact.
The tap selector, as noted, is usually a three-phase system and can operate with an oil-filled housing which has the selector contacts, preselector contacts, the vacuum switching cells and the bypass contacts. The term "preselector contacts" are contacts which can be used optionally for a coarse selection (range selection) or for a possible reversal. The two switching variants are also known in connection with reaction type systems of the kind with which the invention is concerned in the art. In separate housing parts a drive is usually provided for actuating the individual contact and the vacuum switching cells.
In the housing, terminal plates are provided which are separate for each of the three phases to be switched and on which the selector and reversing contacts are provided. Further plates also provided for each phase carrying the corresponding vacuum switching cells and the associated bypass contacts. For example, on one side of such a further plate which is turned toward the corresponding terminal plate, the fixed and movable bypass contacts are provided while on the opposite side the vacuum switching cell with a respective force-storing mechanism for its actuation can be mounted.
Such a force-storing mechanism is described in detail in German patent document 41 26 824.
All of the switch elements of all of the phases can be driven by a single insulated shaft which traverses the lateral housing portions or is connected to a drive mechanism laterally of the housing. It is common in this kind of construction to provide three geneva mechanisms, one for each phase, each of which is mounted on the respective terminal plate. These geneva mechanisms convert the rotary movement of the drive shaft to the intermittent movements required to actuate the selector and reversing contacts as well as the movements for actuating the bypass contacts and for actuating the force storers to trip the corresponding vacuum switching cells in the predetermined switching sequence.
The single insulating shaft thus operates three separate geneva mechanisms and each of these geneva mechanisms actuates the movable elements of a respective terminal plate of the respective phase, namely, the tap selection contacts and via a separate pin on the Geneva mechanism, the reversing contact. Separately for each phase, utilizing a double-sided groove in the rotatable disk, the bypass contacts and the force-storing device for the vacuum-switching cell are actuated. A double-sided cam arrangement of this type is described in German patent document 40 11 019.
In practice it is found that such constructions are relatively complex and subjected to mechanical deterioration or are mechanically unreliable because of jamming or the like. The several geneva mechanisms increase the complexity and since a number of mechanisms are provided which must be cooperated with great precision, the overall fabrication cost of the apparatus is substantial. The double-sided cam for the simultaneous actuation of the bypass contacts and the vacuum-switching cells also contribute to the increased complexity and the problem is rendered more acute because the cam contours are not identical and thus even the fabrication cost for the cam is substantial.